The present invention is directed to methods and apparatus for transmitting signals suitable for use in timing recovery in wireless communication systems, e.g., frequency division multiplexed (FDM) systems.
Orthogonal Frequency Division Multiplexing (OFDM) has emerged as a viable scheme for wireless communications. In OFDM schemes signals are modulated onto multiple carriers in the frequency domain and then transformed, usually via a Fast Fourier Transform (FFT), to produce the time domain version of the signal. One drawback of OFDM systems concerns their high peak-to-average power ratio requirements. This refers to the fact that the transmitted signals in OFDM schemes, when viewed in the time domain, often exhibit large excursions in amplitude. If the transmitter seeks a high fidelity transmission then the dynamic range of the transmitter must be correspondingly large, so as not to distort these large excursions. This has two effects. One is that it increases the cost of power amplifiers required in the transmitter. The second is that signals cannot be transmitted at full power since power margins must be left to allow for occasional large excursions.
Golay sequences are sequences of symbols, e.g., QPSK symbols, that exhibit low peak to average power ratios both in the frequency domain and the time domain when the symbols of the sequence are transmitted using uniformly spaced tones. In the prior art, it was generally assumed that to obtain the benefit of low peak to average power ratios the symbols of the Golay sequence had to be transmitted using uniformly spaced tones in the frequency domain. It has been proposed to use such sequences for transmission in OFDM systems with the sequences being transmitted using a contiguous set of uniformly spaced tones as shown in FIG. 1.
In FIG. 1, the frequency spectrum used to transmit symbols is divided in to a set of N+1 tones, f0 through fN. The set 12 of symbols S0 through S15, of an exemplary sixteen symbol Golay sequence, are transmitted in FIG. 1 using a set 14 of contiguous uniformly spaced tones which are a subset of the full set of utilized tones f0 through fN. Transmitting the symbols of the sequence 12 in this manner provides a signal with low peak to average power ratios. Unfortunately, requiring tones used to transmit the Golay sequence to be uniformly spaced often limits the spectrum over which the sequence of tones will be broadcast, particularly when the number of tones in the sequence is relatively small compared to the total number of tones used in a system.
In many cases, it is desirable to maximize the frequency range over which tones are transmitted. The greater range of frequencies can provide a degree of frequency diversification and increase the ability to accurately measure transmission delays as a function of signal phase shift. Unfortunately, the existing requirement for the tones of a Golay sequence to be transmitted using uniformly spaced tones to obtain the known benefits of such a signal limits the utility of such sequences.
Another difficulty associated with the use of Golay sequences is that there are relatively few Golay sequences so high communication rates are difficult to obtain this way. Nevertheless, there may be certain circumstance in which the use of Golay sequences or Golay like sequences is desirable.
OFDM systems require timing synchronization between the transmitter and receiver. In, e.g., a mobile wireless setting, this normally requires that the mobile transmit a signal that can be used to synchronize the two participants. Timing is typically determined by examining phase offsets of the received signal as a function of frequency. Different frequencies experience different phase offsets as a function of transmission delay. Generally, the greater the difference in frequency between signals, e.g., symbols, used to estimate the transmission delay in this manner, the greater the accuracy of the transmission timing delay estimate.
Another related system function is access. A mobile often requests access to the base station before it begins transmitting its user information. An access signal should indicate to the base station that the mobile is requesting access to that particular base station. The access signal should be recognizable by the base station as an access signal intended for that base station. The signal transmitted by the mobile should therefore indicate the identity of the base station to which it is requesting access. In order to conserve mobile power, minimize out of band transmission, and/or admit high energy transmission of access signals without undue distortion, the access request signal should, preferably, also exhibit a low peak to average power ratio.
In view of the above discussion, it can be appreciated that improved signaling techniques suitable for use in FDM systems, e.g., mobile OFDM systems, are desirable. For example, there is a need for improved signals which can be transmitted and examined to accurately detect transmission timing delays. In addition, there is a need for efficient access request signals, e.g., symbol sequences, with low peak to average ratios which can be associated with different base stations.
While Golay sequences offer some possible advantages with regard to satisfying these two needs, the restriction that the elements of the Golay sequence need to be transmitted using uniformly spaced frequencies needs to be overcome if such sequences or similar sequences are to be highly useful.